I'm not sure about your C Class, but I do know that on a lot of the Mercedes models if you buy a rear facing child seat from Mercedes, they have a transponder built into the seat which automatically detects it and immobilises the passenger front airbag. If you use a seat from anywhere else the airbag will still be active and not suitable for a front seat.
Surely you have an owner's manual with your car, it describes the safety features in there.
If you are in any doubt you should contact your local Merc dealer for advice.